The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, a thin layer of air develops between the slider and the rotating disk. When the slider rides on this air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP1) with a layer of antiferromagnetic material such as PtMn. While an antiferromagnetic (AFM) material such as PtMn does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.
Traditionally, GMR sensors have been constructed as current in plane (CIP) GMR sensors, wherein current flows through the sensor from one side to the other in a direction parallel with the planes of the layers making up the sensor. More recently, increased attention has been focused on current perpendicular to plane (CPP) GMR sensors. As its name suggests, in a CPP sensor, current flows through the sensor from top to bottom in a direction perpendicular to the planes of the layers making up the sensor.
Another type of magnetoresistive sensor is a tunnel magnetoresistance (TMR) sensor or tunnel valve. A tunnel valve includes a pinned layer and a free layer, similar to a GMR sensor. However, instead of having a non-magnetic electrically conductive spacer layer between the free and pinned layers, a tunnel valve has a thin dielectric, non-magnetic barrier layer, which can be constructed of for example alumina Al2O3. A tunnel valve operates based on the spin dependent tunneling of electrons through the thin barrier layer. When the magnetic moments of the free and the pinned layer are aligned parallel with one another, electrons much more readily pass through the barrier layer than when they are the moments are antiparallel. Therefore, current travels through a tunnel valve in a direction perpendicular to the plane of the layers making up the sensor, similar to a current perpendicular to plane (CPP) GMR.
Both CPP GMR sensors and tunnel valves have sense current conducted by first and second leads that generally are constructed of a magnetic material to act as magnetic shields and which contact the top and bottom of the sensor. Both CPP and GMR sensors, therefore, also require the inclusion of some sort of electrical insulation to cover the sides of the sensor as well as at least one of the leads in areas outside of the sensor, in order to prevent current from being shunted between the leads. One process that is currently being used to deposit such a layer of insulation is atomic layer deposition of alumina. This process has the desired ability to apply a thin uniform layer of alumina on both the side vertical surfaces of the sensor as well as the top (horizontal) surface of the bottom lead. This procedure, however, requires the application of high temperatures, which can be problematic for reasons that will be discussed below.
Magnetoresistive sensors have been constructed by a method that includes first depositing the sensor layers as full film layers. Then, a full film layer of CMP resistant material is deposited. A mask that includes one or more layers of a polymer material such as photoresist is then formed to cover the area where the sensor is desired and exposing other portions. An etching process is then used to remove the CMP resistant material and portions of the sensor layers outside of the sensor area.
This process for defining the sensor has various limitations, especially for use in current perpendicular to plane (CPP) sensors, which are the desired sensors for use in future magnetic recording products. As discussed above, the prior art process uses a polymer mask. It turns out that the manufacture of CPP sensors requires a relatively long etch in the case of in-stack stabilized CPP sensors and/or an etch in a harsh reactive ion etch (RIE) chemistry to properly define the sensor stack. This type of etch removes an excessive amount of the polymer mask, which is not sufficiently resistant to the etching process. After the sensor stack has been defined, a fill material such as a dielectric material and/or magnetic material are deposited. A chemical mechanical polishing process (CMP) is used to remove the remaining mask. CMP processes preferentially remove material that has a high topography (ie. sticks up beyond the other structures). If a sufficient amount of mask remains after defining the sensor stack, then the CMP will be able to effectively remove this remaining mask. However, as mentioned above, the polymer mask presently used is not sufficiently resistant to the etching processes currently needed to define the sensor stack. As a result, the excessively thin remaining mask material does not have a high enough topography to be removed by the CMP. It then becomes difficult to remove the mask after the sensor has been defined and the fill material has been deposited.
To make matters worse, processes used to manufacture the desired CPP sensors future generation magnetic heads involve the application relatively high temperatures. For example, the use of atomic layer deposition (ALD) of alumina, which provides a desired conformal layer of alumina Al2O3 requires the application of high temperatures. These high temperatures hard bake the polymer mask, making the mask even more difficult to remove, or worse yet cause the mask to shrivel up and become completely ineffective.
Therefore, there is a strong felt need for a method for defining a sensor stack of a magnetoresistive sensor, wherein the method is compatible with the long etches and/or etching using harsh reactive etch chemistries and high temperature processes needed to construct current and future generation magnetic heads. Such a method would preferably involve the use of a mask that can maintain a sufficient amount of topography after such etching that the mask can be readily removed by CMP. Such a method would also preferably involve the use of a mask that will not be negatively affected by high temperatures needed to construct such current and future generation magnetic heads.